In electronic industry, the well-known PFC (Power Factor Correction) circuit is an important circuit which improves the input power factor used in power supply devices or built in the power supply circuits of various electric appliances. The common PFC circuits may be categorized into the single-stage type and the two-stage type, which are respectively applied to different fields. The two-stage PFC circuit has a higher power factor and a lower total harmonic distortion. The single-stage PFC circuit is simpler and cheaper, however. Either of the two types of PFC circuits should have a storage capacitor, i.e. the so-called bulk capacitor, for energy regulation. For an example of the single-stage PFC circuit, please refer to a R.O.C. patent No. 561675 disclosing a “PFC Circuit with an Oscillation-Damping Circuit”. FIG. 1 of the prior-art patent schematically shows the architecture of a basic PFC circuit. In FIG. 1 of the prior-art patent, the PFC circuit comprises an inductor 107, a diode 108, a capacitor 109 and a switch 106. The capacitor 109 functions as the storage capacitor. An input circuit 101 inputs a pulsed DC power to the single-stage PFC circuit. If the switch 106 is turned on, the power of the pulsed DC power is stored in the capacitor 109. At the same time, the capacitor 109 outputs power to the load 105. While the switch 106 is turned off, the inductor 107 transfers power to the capacitor 109. Such an operation mode generates a modulated power to the load 105. Since the technology of the PFC circuit is a prior art familiar to the people having ordinary knowledge in the related fields, it will not repeat herein.
To achieve the standard of IEC 1000-3-2 (International Electrotechnical Commission), the bus voltage Vbus (the voltage of the main power transmission path) of the abovementioned power supply device should have a small low-frequency ripple. To achieve a high power factor, the bus voltage Vbus should be raised. In the conventional technology, only an electrolytic capacitor can realize the abovementioned objectives. Therefore, electrolytic capacitors are generally used in PFC circuits. Further, the conventional PFC circuit is operated in a discontinuous current mode (DCM) to increase the utility rate of the iron core of the transformer and promote the stability of single-loop control.
However, the electrolytic capacitor has a shorter lifetime. A solid electrolytic capacitor has a lifetime of only thousands of hours at a temperature of 105° C., and a liquid one has a further shorter lifetime. When an electrolytic capacitor is used as the storage capacitor of a PFC circuit, the lifetime of the electrolytic capacitor directly limits the lifetime of the PFC circuit. For example, in an LED driving circuit, LED itself has a lifetime of at least one hundred thousands of hours. However, since the reduction of the storage capacitor, the PFC circuit of the LED driving circuit can only work for thousands of hours (the storage capacitor has an average lifetime of only thousands of hours). Thus, when the storage capacitor breaks down, the circuit board of the PFC circuit and the LEDs soldered on the circuit board have to be replaced at the same time. Thus, the LEDs should be abandoned even before one half of the lifetime thereof has elapsed, which is indeed a waste of the resource. Therefore, it is very deserving to solve the problem that the lifetime of a PFC circuit is limited by the lifetime of the storage capacitor thereof.